I had this idea for a summer project to keep me entertained:

I have been reading about making home stereo speakers.

I want to do this using car speakers, not traditionally hi-fi speakers,
anyone tried it?

I like the look of Kenwood and Sony models, and I have my eye on the
KFC-1758S and XS-V1633.

It's just an idea for a project for the summer. I have a stereo amp which
will switch between 4 (cars) and 8 (Hi-Fi) ohms. I want to use this to
drive
car speakers but
someone has told me that it won't work as cars fun on 12 volts (which I
knew). But thinking
about it and applying Ohms law (V = I x r) I don't see why the 12 volts
supplied by the battery
matters.

It matters because it simply will not turn on if you provide 12 volts DC to
an amplifier that needs 110 volts AC to run.

I am aware of the 4 ohm (car) and 8 ohm (hi-fi) difference. Any other
problems? Anyone have any experience?

Also, are all car speakers 4 ohms out of interest?

No, not all. But most.